1. Field of the Invention
The present invention relates to cellular wireless devices. More specifically, a method is disclosed for performing initial and target base station acquisition for a direct sequence code division multiple access (DS/CDMA) system.
2. Description of the Prior Art
Spread spectrum communication systems are becoming increasingly important in cellular networks. In particular, so-called third generation (3G) cellular standards have adopted direct sequence code division multiple access (DS/CDMA) as a communications standard.
To establish a network connection in a CDMA system, the user equipment (UE) must first perform a cell search procedure. The cell search procedure enables the UE to obtain timing and code synchronization for the downlink channel. Various methods are known in the prior art for performing a cell search procedure. Attention is drawn, for example, to the article “Cell Search in W-CDMA” by Yi-Pin Eric Wang and Tony Ottosson in Vol. 18, No. 8 (August 2000 edition) of IEEE Journal on Selected Areas in Communications, which is included herein by reference. Base station acquisition is also discussed in U.S. Pat. No. 6,363,060 to Sarkar; U.S. Pat. No. 5,930,366 to Jamal et al., and U.S. Pat. No. 6,226,315 to Sriram, all of which are included herein by reference.
A simple overview of cell searching is presented in the following. Please refer to FIG. 1. FIG. 1 is a block diagram of a downlink Common Control Channel (CCH) 10 in a DS/CDMA system. The CCH 10 is broken up into a series of frames 12. Each frame 12 contains fifteen slots 14. Each slot 14 holds ten symbols, each of 256 chips. Hence, each slot 14 is 2560 chips in length. Please refer to FIG. 2 in conjunction with FIG. 1. FIG. 2 is a block diagram of a slot 14 in the CCH 10. The first symbol 16 in each slot 14 holds a primary synchronization channel (PSCH) 16p and a secondary synchronization channel (SSCH) 16s. The remaining nine symbols 18 follow after the first symbol 16. The PSCH 16p and SSCH 16s are orthogonal to each other, and hence can be broadcast on top of each other. The PSCH 16p chip coding is the same for all base stations, and does not change. The SSCH 16s chip coding changes with every slot 14 according to a predefined pattern that repeats every frame 12. Please refer to FIG. 3. FIG. 3 is a block diagram of a common pilot channel (CPICH) 20 broadcast with the CCH 10. The coding used for the CPICH 20 is unique to the broadcasting base station. In a DS/CDMA system, a base station can use one of 512 different codes for the CPICH 20, which are broken into 64 code groups, each having 8 respective codes. The coding of the PSCH 16p is common across all base stations, and can thus be used for slot 14 synchronization. Although the coding of the SSCH 16s changes on a slot 14 by slot 14 basis, the sequence pattern of code change of the SSCH 16s is determined by the code group into which the code used for the CPICH 20 lies. That is, there are 64 code sequence patterns for the SSCH 16s to follow, each of which corresponds to a particular code group associated with the code used for the CPICH 20. By correlating the received CCH signal 10 with all possible SSCH 16s code sequences and identifying the maximum correlation value, it is possible to learn the code group of the CPICH 20, and to obtain frame 12 synchronization. This is due to the fact that the SSCH 16s changes according to a predefined sequence, the starting sequence of which is known and which is sent at the beginning of every frame 12, thus enabling frame synchronization. Once the code group of the CPICH 20 is learned, it is possible to obtain the primary scrambling code used by the cell by performing symbol-by-symbol correlation over the CPICH 20 with all eight of the codes in the code group identified for the CPICH 20. Once the primary scrambling code used by the base station has been identified, system and cell specific broadcast channel (BCH) information can be read.
Based upon the above, cell searching is thus typically broken into the three following steps:
Step 1: Slot synchronization.                Utilize the PSCH 16p to perform slot synchronization. This is typically done with a matched filter (or similar device) that is matched to the PSCH 16p that is common to all base stations. Slot timing is obtained from peaks in the matched filter output.        
Step 2: Frame synchronization and code group identification.                The slot timing obtained in step 1 is used to correlate the SSCH 16s with all possible SSCH code sequences. The maximum correlation identifies the code group of the CPICH 20. The SSCH 16s having the first SSCH code sequence identifies the start of a frame 12.        
Step 3: Scrambling code identification.                Symbol-by-symbol correlation is performed on the CPICH 20 for all eight codes within the code group identified in step 2. The maximum correlation value identifies the primary scrambling code of the base station.        
Please refer to FIG. 4. FIG. 4 is a simple block diagram that illustrates cell synchronization for a prior art UE 30. Of course, the UE 30 will contain many more components than those shown in FIG. 4, which is restricted to the present discussion. The UE 30 includes a transceiver 39 and a synchronization stage 38. The transceiver 39 receives broadcasts from a base station (not shown) and passes broadcast data to the synchronization stage 38 in a manner familiar to those in the art of wireless devices. The synchronization stage 38 includes a stage 1 31, a stage 2 32 and a stage 3 33. The stage 1 31 performs the slot synchronization of step 1 discussed above. Results from stage 1 31 are passed to stage 2 32, which performs the frame 12 synchronization and code group identification of step 2. Results from stage 2 32 are then passed on to stage 3 33, which performs the scrambling code identification of step 3. Stage 1 31 includes a peak profiler 34. The peak profiler 34 contains the primary synchronization code 35 that is common to all base stations, and generates peak profile data 36 that is obtained by matching the primary synchronization code 35 against the PSCH 16p received from the transceiver 39. The profile data 36 holds data for a predetermined number of chips, and as the PSCH 16p repeats with every slot 14, it is common to hold enough data to cover an entire slot 14, i.e., 2560 chips. The chip in the profile data 36 having the highest peak profile is assumed to mark the PSCH 16p, and is thus used as the PSCH path position 37. This is illustrated in FIG. 5, which is an example graph of peak profile data 36 (not to scale). Stage 1 31 notes that in the profile data 36 a maximum valued peak occurs at chip number 1658. The PSCH path position 37 would thus hold a value indicative of the peak path position at chip 1658. The PSCH path position 37 is forwarded to stage 2 32 as the slot 14 synchronization point. Utilizing the slot 14 position marked by the PSCH path position 37, stage 2 32 performs step 2 outlined above to generate a code group value 32g. Stage 2 32 will also generate a slot number 32s, which indicates the number of the slot 14 in its respective frame 12 that was marked by the PSCH path position 37. As there are fifteen slots 14 in a frame 12, the slot number 32s could be a value that runs, for example, between zero and fourteen. In this manner, frame 12 synchronization is performed. Finally, the results from stage 2 32 are passed on to stage 3 33, which subsequently performs step 3 to generate a primary scrambling code 33p for the CPICH 20. A verification stage 38v can be used to verify the results obtained from stage 3 33, and typically involves a process similar to that performed in stage 3 33, but with stricter correlation parameters that are performed over more frames 12.
Typically, two types of cell searching is done: an initial cell search when the UE 30 is first turned on, and a target cell search when the UE 30 is attempting to identify local base stations. In an initial cell search, the UE 30 attempts to find a base station having the best reception, and thus will choose the maximum peak present in the profile data 36. However, in a target cell search, successively smaller peaks are checked, passing through stage 1 31, stage 2 32 and stage 3 33 to obtain the corresponding primary scrambling codes of these other base stations. This process is both time consuming and power intensive. In a target cell search, the prior art synchronization stage 38 will examine the “n” highest peaks in the profile data 36. For example, the six highest peaks may be checked, and respectively run through the synchronization stage 38. A drawback to this, though, is that multi-path components are also found. For example, in FIG. 5, the peak a chip number 1661 may be a multi-path component of the peak at path position 1658. Running a multi-path component through the three stages of the synchronization stage 38 consumes both time and battery power.